Vehicles may be configured with a starter motor for spinning an engine from rest. In some vehicle systems, the starter motor may be included in a belt-driven integrated starter generator (BISG) where an electric motor/generator (acting in the motoring mode) provides torque to crank the engine via a belt drive during engine restart conditions. During other conditions, the motor/generator may be operated in the generating mode to charge a system battery using excess engine torque. In doing so, the BISG enables stop/start functions to be provided. The BISG may also be used in place of an alternator to maintain accessory load functions when the engine is shut down and the vehicle is stopped. Further, the BISG may be used to provide a hybrid power assist to the engine torque by adding motor torque drawn from the battery. In this way, use of a BISG can improve vehicle performance and fuel economy.
BISG systems may include one or more tensioners for maintaining tension in the belt drive coupling the BISG to an engine crankshaft and one or more accessories such as an air conditioning system. Belt tension may be maintained by way of passive tensioners including, for example, various pulleys and biased springs with integral dampers. Alternatively, belt tension may be provided by way of actively controlled tensioner devices, such as electric, hydraulic, or electro-hydraulic devices, to selectively increase belt tension only when high torque demands are made. Belt tensioners may be periodically diagnosed so as to enable belt drive tension to be maintained, thereby reducing friction losses.
One example approach for diagnosing the functionality of a tensioner is shown by Zettel et al. in US20080021603. Therein, a belt drive couples the motor generator to each of an engine and an accessory, and a degraded state of the belt drive is inferred based on a comparison of engine speed relative to accessory speed. In particular, if an expected accessory speed, determined based on the measured engine speed differs, from the actual accessory speed, it may be determined that there is belt slippage.
However, the inventors herein have identified potential issues with such an approach. As one example, by the time belt slippage is detected by the method of Zettel, there may already be excessive slip, requiring a costly replacement of the belt. As another example, the approach of Zettel is not able to identify the cause of belt slippage. Belt slippage may be caused due to insufficient tension in the belt, which may degrade engine crank and torque boost events. Alternatively, belt slippage may be caused due to elevated tension in the belt under all conditions, which can cause increased engine and belt wear. If the cause of belt slippage is not clearly determined, it may be difficult to mitigate future belt slip events. In the case of belt tensioners coupled to a BISG, distinction of a cause of the belt slippage based on engine speed or accessory speed may be further complicated by the ability of the BISG to increase tension by adding positive torque or negative torque to the belt drive during a torque transient, e.g. a sudden increase or decrease in BISG torque.
The inventors herein have recognized the above-mentioned issues and have developed an engine operating method to identify degradation of an active belt tensioner. One example method for an engine comprises: adjusting an operating state of a belt tensioner coupled to a drive belt; adjusting an operating state of a belt tensioner coupled to a drive belt; and indicating tensioner degradation in response to engine load being less than a threshold after the adjusting. In this way, a cause of belt slippage may be reliably identified before excessive slip occurs, allowing for mitigating actions to be timely performed.
As an example, a hybrid vehicle system may include an engine coupled to a BISG and an accessory via a front end accessory belt drive (FEAD). The belt drive may further include an active tensioner coupled between the engine crankshaft and the BISG. The active tensioner may be an electro-hydraulic active tensioner configured to operate at a lower tension when the BISG torque is below a threshold and at a higher tension when positive or negative torque provided by the BISG is below a different threshold. In particular, during an engine start, an electric solenoid may be energized to actuate the tensioner to an extended state where the tensioner applies a higher tension to the belt drive. The higher tension enables positive torque to be transferred from the BISG, operating as a motor, to crank the engine. During an engine start/stop event, the electric solenoid may again be energized to actuate the tensioner to the extended state. The higher tension may enable negative torque to be transferred from the BISG, operating as a generator, to stop the engine, while damping the engine torque pulsations. During all other engine operating conditions, the electric solenoid may be de-energized to actuate the tensioner to a retracted state where the tensioner applies a lower tension to the belt drive. During selected conditions, such as when the engine is in a steady-state and the BISG torque is at or around zero torque, the tensioner may be actively actuated to the extended state and one or more engine torque parameters indicative of an increase in engine load may be monitored over a duration. For example, one or more of an intake air flow, an intake manifold air pressure, a rate of engine fuel consumption, and an engine torque output may be monitored. If the monitored parameters are within an expected range, the measured increase in engine load is attributed to the tensioner extending and applying an increased tension to the belt drive, as intended. If the monitored parameters are below the expected range, then it may be inferred that the lack of increase in engine load is due to the tensioner not extending (e.g., the tensioner was stuck in the retracted state) and that belt slip may occur due to insufficient belt tension. Optionally, the tensioner may be similarly actuated to the retracted state, and it may be inferred that the tensioner did not retract (e.g., tensioner was stuck in the extended state) if the monitored parameters are above the expected range, and further that belt slip may occur due to excessive belt tension. Accordingly, distinct mitigating actions may be performed. For example, in response to the tensioner being stuck in the retracted state, power assist provided by the BISG and start/stop operations may be limited. As another example, in response to the tensioner being stuck in the extended state, boosted engine torque may be limited to prevent damage to the FEAD belt and engine journal bearings.
In this way, an active tensioner of a BISG may be accurately and reliably diagnosed, enabling belt slip to be better predicted. The technical effect of actuating the active tensioner to a selected state and monitoring a change in FEAD load following the actuation is that the actual operating state of the tensioner may be better identified. By using the monitored change in FEAD load to compare the actual state of the tensioner relative to the commanded state, the presence of excess belt tension may be distinguished from the presence of insufficient belt tension. Consequently, incidence of belt slip and the likely cause of belt slip may be identified in a timely manner, before excessive slip occurs. By timely diagnosing a belt tensioner, belt health may be improved, enabling the fuel economy benefits of a BISG system to be extended.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.